| Foliar blight and root rot of American ginseng (Panax quinquefolium ) caused by Phytophthora cactorum has been managed with the fungicide mefenoxam. During 2003 and 2004, commercial ginseng gardens were monitored in Wisconsin (WI) and Michigan (MI) as well as research gardens at Michigan State University (MSU). Following periods of cool, wet weather, reddening of leaflets, root rot, plant wilting and death were observed. Diseased plants were sampled and P. cactorum was isolated. A minimum of 104 P. cactorum isolates were recovered each year. Isolates were screened for sensitivity to mefenoxam by measuring radial mycelial growth on 100-ppm mefenoxam-amended V8 agar. For 2003, 79, 48 and 85% of isolates collected from MSU, MI and WI plants were resistant to mefenoxam, respectively. In 2004, 83 and 91% of isolates recovered from MSU and WI plants and seedlings were resistant to mefenoxam, respectively. Isolates were tested for virulence using an apple fruit bioassay. All were virulent. Alternative products were tested against P. cactorum. Mefenoxam was the least effective while dimethomorph, mancozeb with zoxamide and copper hydroxide were the most effective against P. cactorum. Phytophthora cactorum is an important pathogen in MI and WI gardens. The development of isolates resistant to mefenoxam is a new finding. Currently un-registered fungicides may be useful in managing P. cactorum diseases. |
